This invention relates to new liquid crystalline compounds and liquid crystal compositions. More specifically, this invention relates to the liquid crystalline compounds having difluoromethyleneoxy moiety, liquid crystal compositions comprising the compounds, and liquid crystal display devices fabricated by using the liquid crystal composition.
Liquid crystal display devices fabricated by using liquid crystalline compounds (the term xe2x80x9cliquid crystalline compoundsxe2x80x9d is used in this specification as a generic term for the compounds which have a liquid crystal phase and the compounds which do not have a crystal phase but are useful as a component of liquid crystal compositions) have widely been used for the display of computers, television sets, and the like.
For the purpose of reducing electric power consumption and decreasing the leakage of an electromagnetic wave, the liquid crystal compositions are required to lower their driving voltage. The driving voltage (threshold voltage) is known to be a function of a dielectric anisotropy value and an elastic constant as is shown in the following equation (M. F. Leslie, Mol. Cryst. Liq. Cryst., 12, 57 (1970)):
Vth=xcfx80(K/∈0xcex94∈)xc2xd
wherein Vth is threshold voltage, ∈0 is a dielectric constant in vacuum, K is an elastic constant, and xcex94∈ is dielectric anisotropy.
That is, in order to lower the driving voltage, it is required 1) to increase the dielectric anisotropy value, and 2) to decrease the elastic constant. It is generally difficult to adjust the elastic constant value of the liquid crystalline compounds, and a method of enlarging the dielectric anisotropy value is principally employed for lowering the driving voltage. Accordingly, new liquid crystalline compounds having a large value of dielectric anisotropy have been expected.
From some time ago, a most serious problem on the liquid crystal display devices is a narrow viewing angle, and various display modes have been proposed in recent years for the purpose of improving it. In-plane switching (IPS) display devices proposed in 1995 greatly widened the viewing angle compared with conventional display devices (Liquid Crystal Conference in Japan 2A07 (1995), ASIA DISPLAY ""95, 557(1995), and ASIA DISPLAY ""95, 707 (1995)). The characteristic of this mode is that various liquid crystal compositions can be utilized regardless of positive or negative in the dielectric anisotropy value.
In 1997, an attempt utilizing a vertical alignment (VA) cell was reported (SID 97 DIGEST, 845 (1997) and the display devices of this mode has also considerably wide viewing angle compared with conventional display devices. The characteristic of this mode is that the liquid crystal compositions having a negative dielectric anisotropy value can be utilized.
As a compound having a negative and large dielectric anisotropy value and a comparatively small optical anisotropy value, compound (13) in the following has been known (V. Reiffenrath et al., Liq. Cryst., 5 (1), 159 (1989)). It is reported that the dielectric anisotropy value (xcex94∈) of this compound is xe2x88x924.1 and the optical anisotropy value (xcex94n) is 0.18. 
Whereas compound (13) has a large dielectric anisotropy value, an optical anisotropy value is large. Then, it could not satisfy the values required for IPS mode or VA mode.
According to Maier and Meier, a dielectric anisotropy value and an optical anisotropy value are regarded as a function of the order parameters (W. Maier and G. Meier, Z. Naturf. (a), 16, 262 (1961)). Namely, it is possible to decrease the optical anisotropy value by the decrease of the order parameters. As the optical anisotropy value and the dielectric anisotropy value are generally proportional in a single liquid crystal molecule, decreasing the optical anisotropy value will cause a decrease of the dielectric anisotropy value. Therefore, a new compound having a unique property which decreases only the optical anisotropy value without a large decrease of the dielectric anisotropy value has been expected.
Compound (14) having cyano has also been known as a compound having a small optical anisotropy value (R. Eidenschink et al., Angew. Chem., 96, 151 (1984)). It has both a large dielectric anisotropy value and a small optical anisotropy value. However, the liquid crystal compositions containing the compound for IPS mode and VA mode have considerably low contrast, as specific resistance is small and temperature dependence of a voltage holding ratio is very large. Thus, such compound is unsuitable as liquid crystalline compound in the use of the liquid crystal composition for IPS mode or VA mode. 
In view of the several characteristics required for the liquid crystal compositions described above, the object of this invention is to provide liquid crystalline compounds having a negative and extremely large dielectric anisotropy value and a small optical anisotropy value at the same time, to provide liquid crystal compositions comprising the compound, and to provide liquid crystal display devices fabricated by using the liquid crystal composition.
As a result in the diligent research and development of the present inventors to solve the subject described above, they found that the compounds with two rings, three rings or four rings, having both a moiety in which two rings are bridged by xe2x80x94CF2Oxe2x80x94 bonding group and a moiety of 2,3-difluoro-1,4-phenylene at the same time, have not only a negative and large dielectric anisotropy value but also are excellent in miscibility with other compounds, have high specific resistance and a high voltage holding ratio, and are stable physically and chemically, and then they completed this invention.
Namely, this invention contains items [1] to [22].
[1] A liquid crystalline compound expressed by formula (1) 
wherein Ra and Rb each independently represents a linear or branched alkyl having 1 to 10 carbon, any methylene in the alkyl may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or xe2x80x94Cxe2x89xa1Cxe2x80x94, but xe2x80x94Oxe2x80x94 is not successive, and any hydrogen in the alkyl may be replaced by halogen; rings A1 to A4 each independently represents trans-1,4-cyclohexylene, cyclohexene-1,4-diyl, pyridine-1,4-diyl, pyrimidine-2,5-diyl, or 1,4-phenylene, wherein at least one hydrogen in these rings may be replaced by halogen, and any nonadjacent methylene in cyclohexane ring may be replaced by xe2x80x94Oxe2x80x94; Y1 and Y2 each independently represents F or Cl; Z1, Z2 and Z3 each independently represents a single bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CF2Oxe2x80x94, or xe2x80x94OCF2xe2x80x94; and l, m and n each independently is 0, 1 or 2, and the sum of l+m+n is 3 and less.
[2] A liquid crystalline compound described in item [1], wherein in formula (1), Y1 and Y2 are fluorine.
[3] A liquid crystalline compound described in item [1], wherein in formula (1), Y1and Y2 are fluorine and ring A2 is trans-1,4-cyclohexylene.
[4] A liquid crystalline compound described in item [1], wherein in formula (1), Y1 and Y2 are fluorine and ring A2 is cyclohexene-1,4-diyl.
[5] A liquid crystalline compound described in item [1], wherein in formula (1), Y1 and Y2 are fluorine and ring A2 is 1,4-phenylene in which hydrogen in the ring may be replaced by fluorine.
[6] A liquid crystalline compound described in item [1], wherein, in formula (1), l=1, m=n=0, ring A1 and ring A2 are trans-1,4-cyclohexylene, Z1 is a single bond, and Y1 and Y2 are fluorine.
[7] A liquid crystalline compound described in item [1], wherein in formula (1), l=1, m=n=0, ring A1 is trans-1,4-cyclohexylene, and ring A2 is cyclohexene-1,4-diyl, Z1 is a single bond, and Y1and Y2 are fluorine.
[8] A liquid crystalline compound described in item [1], wherein in formula (1), l=1, m=n=0, ring A1 is trans-1,4-cyclohexylene, ring A2 is 1,4-phenylene in which hydrogen in the ring may be replaced by fluorine, Z1 is a single bond, and Y1 and Y2are fluorine.
[9] A liquid crystalline compound described in item [1], wherein in formula (1), l=n=0, m=1, ring A2 is trans-1,4-cyclohexylene, A3 is 1,4-phenylene in which hydrogen in the ring may be replaced by fluorine, Z2 is a single bond, and Y1 and Y2 are fluorine.
[10] A liquid crystalline compound described in item [1] wherein in formula (1), l=n=0, m=1, ring A2 is cyclohexene-1,4-diyl, ring A3 is 1,4-phenylene in which hydrogen in the ring may be replaced by fluorine, Z2 is a single bond, and Y1 and Y2 are fluorine.
[11] A liquid crystalline compound described in item [1], wherein in formula (1), l=m=1, n=0, and Y1 and Y2 are fluorine.
[12] A liquid crystalline compound described in item [1] wherein in formula (1), l=2, m=n=0, and Y1 and Y2 are fluorine.
[13] A liquid crystal composition characterized in that it comprises at least one of the liquid crystalline compounds described in any one of items [1] to [12].
[14] A liquid crystal composition characterized in that it comprises, as a first component, at least one liquid crystalline compound described in any one of items [1] to [12], and comprises, as a second component, at least one compound selected from the group of compounds consisting of formulae (2), (3) and (4). 
wherein R1 represents alkyl having 1 to 10 carbon, any nonadjacent xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and any hydrogen in the alkyl may be replaced by fluorine; X1 represents fluorine, chlorine, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H or xe2x80x94OCF2CFHCF3; L1 and L2 each independently represents hydrogen or fluorine; Z4 and Z5 each independently represents xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond; ring A and ring B each independently represents trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl or 1,4-phenylene in which hydrogen may be replaced by fluorine, ring C represents trans-1,4-cyclohexylene or 1,4-phenylene in which hydrogen may be replaced by fluorine.
[15] A liquid crystal composition characterized in that it comprises, as a first component, at least one liquid crystalline compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (5) and (6). 
wherein R2 and R3 each independently represents alkyl having 1 to 10 carbon, any nonadjacent xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and any hydrogen in the alkyl may be replaced by fluorine; X2 represents xe2x80x94CN or xe2x80x94Cxe2x89xa1Cxe2x80x94CN; ring D represents trans-1,4-cyclohexylene, 1,4-phenylene, 1,3-dioxane-2,5-diyl, or pyrimidine-2,5-diyl; ring E represents trans-1,4-cyclohexylene, 1,4-phenylene in which hydrogen may be replaced by fluorine, or pyrimidine-2,5-diyl; ring F represents trans-1,4-cyclohexylene or 1,4-phenylene; Z6 represents xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or a single bond; L3, L4 and L5 each independently represents hydrogen or fluorine; b, c and d each independently 0 or 1.
[16] A liquid crystal composition characterized in that it comprises, as a first component, at least one liquid crystalline compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (7), (8) and (9). 
wherein R4 and R4 each independently represents alkyl having 1 to 10 carbon, any nonadjacent xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and any hydrogen in the alkyl may be replaced by fluorine; ring G and ring I each independently represents trans-4-cyclohexylene or 1,4-phenylene; L6 and L7 each independently represents hydrogen or fluorine but both of them are not hydrogen at the same time; Z7 and Z8 each independently represents xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond.
[17] A liquid crystal composition characterized in that it comprises, as a first component, at least one liquid crystalline compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (2), (3) and (4) as described above, and as a third component, at least one compound selected from the group of compounds consisting of formulae (10), (11) and (12). 
wherein R6 and R7 each independently represents alkyl having 1 to 10 carbon, any nonadjacent xe2x80x94CH2xe2x80x94 in the alkyl may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94, and any hydrogen in the alkyl may be replaced by fluorine; ring J, ring K and ring M each independently represents trans-1,4-cyclohexylene, pyrimidine-2,5-diyl, or 1,4-phenylene in which hydrogen may be replaced by fluorine; Z9 and Z10 each independently represents xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94(CH2)2-, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond.
[18] A liquid crystal composition characterized in that it comprises, as a first component, at least one liquid crystalline compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (7), (8) and (9) as described above, and as a third component, at least one compound selected from the group of compounds consisting of formulae (10), (11) and (12) as described above.
[19] A liquid crystal composition characterized in that it comprises, as a first component, at least one compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (2), (3) and (4) as described above, and as a third component, at least one compound selected from the group of compounds consisting of formulae (7), (8) and (9) as described above.
[20] A liquid crystal composition characterized in that it comprises, as a first component, at least one compound described in any one of items [1] to [12], and as a second component, at least one compound selected from the group of compounds consisting of formulae (2), (3) and (4) as described above, and as a third component, at least one compound selected from the group of compounds consisting of formulae (5) and (6) as described above, and as a fourth component, at least one compound selected from the group of compounds consisting of formulae (7), (8) and (9) as described above.
[21] A liquid crystal composition characterized in that it further comprises one or more optically active compound in addition to the liquid crystal composition described in any one of items [13] to [20].
[22] A liquid crystal display device fabricated by using the liquid crystal composition described in any one of items [13] to [21].
In the liquid crystalline compounds expressed by formula (1), the following compounds expressed by formulae (1-1) to (1-6) have especially suitable characteristics.
In these formulae, Ra, Rb, rings A1, A2, A3, and A4 have the same meaning as described above. 
In formula (1), Ra and Rb are preferably alkyl, alkoxy, alkoxyalkyl, alkoxyalkoxy, alkenyl, alkynyl, alkenyloxy, halogen-substituted alkyl, halogen-substituted alkenyl, halogen-substituted alkenyloxy, halogen-substituted alkoxy, halogen-substituted alkynyl and the like. Compounds having alkyl, alkoxy, alkoxyalkyl, alkoxyalkoxy, halogen-substituted alkyl or halogen-substituted alkoxy as Ra and Rb, are chemically stable, while compounds having alkenyl or alkynyl as Ra and Rb, have fairly large optical anisotropy.
Compounds having, alkyl, alkoxy or alkenyl as Ra and Rb are preferable due to their low viscosity. In the use for display devices of IPS mode or VA mode, compounds having alkyl or alkoxy as Rb are most preferable because high chemical stability and a large dielectric anisotropy value are required.
In formula (1), Z1, Z2 and Z3 are preferably a single bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CF2Oxe2x80x94 or xe2x80x94OCF2xe2x80x94. When Z1, Z2 and Z3 each independently is a single bond, such compounds have a nematic phase over the wide range of temperature together with low viscosity.
Y1 and Y2 represent fluorine or chlorine, and fluorine is more preferable because such compounds have larger negative dielectric anisotropy. Especially when both Y1 and Y2 are fluorine, such compounds have the largest negative dielectric anisotropy value.
Rings A1, A2, A3, and A4 each independently is preferably trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, tetrahydropyrane-2,5-diyl, 1,4-phenylene, or fluorine-substituted 1,4-phenylene, pyridazine-3,6-diyl, pyridine-2,5-diyl, or pyrimidine-2,5-diyl, and 1,4-cyclohexylene is most preferable to obtain compounds with low viscosity. Compounds which ring A1, A2 or A3 is 1,3-dioxane-2,5-diyl are preferable because they have has a small elastic constant value (K) and they lower driving voltage in the liquid crystal display devices with TN mode including IPS mode and VA mode. Compounds which ring A1, A2, A3 or A4 is fluorine-substituted 1,4-phenylene, pyridazine-3,6-diyl, pyridine-2,5-diyl, or pyrimidine-2,5-diyl have a larger dielectric anisotropy value.
The compounds expressed by formula (1) of this invention can be produced by the suitable combination of known procedures of organic synthetic chemistry. The known procedures of organic chemistry can be found by referring to books such as Organic Synthesis, Organic Reactions, and Shin-Jikken Kagaku Kouza (Course of New Chemical Experiment). Their typical examples are shown below.
Compound (3) of formula (1) having ester as a bonding group can be synthesized by reacting various phenol or alcohol (2) with carboxylic acid derivative (1) which can be produced by a known method in the literature, and by carrying out a dehydrating condensation. The dehydrating condensation can be preferably conducted by a known dehydrating agent (for example, dicyclohexylcarbodiimide etc.). It can also be preferably produced by leading carboxylic acid (1) into acid anhydride followed by reacting with phenol or alcohol (2) under the basic conditions.
The ester derivative (3) can be derived to compound (4) by a known sulfurizing agent such as Lawesson""s reagent. Compound (5) of formula (1) in which a bonding group is OCF2 can be synthesized by fluorinating thiocarbonyl of compound (4) by a fluorinating reagent such as hydrogen fluoride-pyridine (M.Kuroboshi et al., Chem. Lett., 827, 1992) or diethylaminosulphur trifluoride (William H. Bunnelle et al., J. Org. Chem., 1990, 55, 768).
Compound (8) in which ring A2 is 1-cyclohexene-1,4-diyl and a bonding group is ester in formula (1) can be synthesized by deriving carboxylic acid derivative (1) which can be produced by a known procedure in the literature, into its acid chloride (6) and by reacting with trimethylsilyl enol ether (7) in the presence of mercury chloride (II) (Kenneth J. Shea et al., J. Am. Chem. Soc. 1987, 109, 447).
The ester derivative (8) can be derived to compound (9) by use of a known sulfurizing agent such as Lawesson""s reagent. Compound (10) in which a bonding group is OCF2 in formula (1) can be synthesized by fluorinating thiocarbonyl of compound (9) using a fluorinating agent. 
The introduction of 1,3-dioxane-2,5-diyl to the moiety of rings A1, A2, A3 and A4 can be conducted according to the method of H. M. Vorbrodt or R. Eidenschink (H. M. Vorbrodt, J. Prakt. Chem. , 323, 902 (1981); R. Eidenschink, DExe2x80x94OS-3306960 (1983)).
As the liquid crystalline compounds of this invention thus obtained have a large negative dielectric anisotropy value, low voltage driving of the liquid crystal display devices can be realized.
The liquid crystalline compounds are superior as a component of nematic liquid crystal compositions, as they are sufficiently stable physically and chemically under the conditions where the liquid crystal display devices are ordinarily used, they can be readily mixed with various liquid crystal materials, and they have excellent miscibility even at low temperatures.
As these compounds have a large negative dielectric anisotropy value together with relatively a small optical anisotropy value, they can most preferably be used as a component of the liquid crystal compositions for IPS mode and VA mode.
Followings are an explanation of the liquid crystal composition of this invention. The liquid crystal composition provided by this invention may contain only a first component comprising at least one compound expressed by formula (1). In addition to the first component, it is preferable to mix, as a second component, at least one compound (hereafter defined as a second component A) selected from the group of compounds expressed by above-mentioned formulae (2), (3) and (4) and/or at least one compound (hereafter defined as a second component B) selected from the group of compounds expressed by formulae (5) and (6). Further, in order to adjust threshold voltage, a temperature range of a liquid crystal phase, an optical anisotropy value, a dielectric anisotropy value, and viscosity, at least one compound selected from the group of compounds expressed by formulae (10), (11) and (12) can be mixed as a third component. Each component of the liquid crystal composition used in this invention may be a relative compound containing corresponding isotope elements, as there is no significant difference in their physical properties.
Among the second component A, (2-1) to (2-9) for the preferable examples of compounds expressed by formula (2), (3-1) to (3-94) for the preferable examples of compounds expressed by formula (3), and (4-1) to (4-33) for the preferable examples of compounds expressed by formula (4) can be illustrated, respectively. 
wherein R1 and X1 have the same meaning as described above.
These compounds expressed by formulae (2) to (4) are principally used for the liquid crystal compositions for TFT because of a positive dielectric anisotropy value and excellent thermal and chemical stability. In case of preparing the liquid crystal composition for TFT, the amount of the compound is suitable in the range of 1 to 99% by weight, preferably 10 to 97% by weight, more preferably 40 to 95% by weight, based on the total weight of the liquid crystal composition. The compounds expressed by formulae (10) to (12) may be further added to adjust viscosity.
Then, as the preferable example of the compounds expressed by formulae (5) and (6) in the second component B, (5-1) to (5-40) and (6-1) to (6-3) can be illustrated, respectively. 
wherein R2, R3 and X2 have the same meaning as described above.
The compounds expressed by formula (5) and (6) are used principally for the liquid crystal compositions for STN and TN, because their dielectric anisotropy values are positive and the values are extremely large. These compounds are used as a component of the compositions especially with an aim to lower threshold voltage. Also they can be used for the object to adjust viscosity, to adjust an optical anisotropy value, and to widen a temperature range of a liquid crystal phase, or for the further object to improve steepness. In case of preparing the liquid crystal compositions for STN or TN, the amount of compounds expressed by formulae (5) and (6) is suitable in the range of 0.1 to 99.9% by weight, preferably 10 to 97% by weight, and more preferably 40 to 95% by weight. A third component described later can be mixed for the object of adjusting threshold voltage, a temperature range of a liquid crystal phase, an optical anisotropy value, an dielectric anisotropy value, viscosity or the like.
In case of preparing liquid crystal compositions having a negative dielectric anisotropy value, which are used for vertical alignment mode (VA mode) and so on, it is preferable to mix at least one compound (hereafter referred to as a second component C) selected from the group of compounds expressed by formulae (7) to (9). Preferable examples of the compound expressed by formulae (7) to (9) of a second component C can be illustrated as (7-1) to (7-3), (8-1) to (8-5), and (9-1) to (9-3), respectively. 
wherein R4 and R5 have the same meaning as described above.
The compounds expressed by formulae (7) to (9) have a negative dielectric anisotropy value. The compounds expressed by formula (7) are principally used for the object of adjusting threshold voltage, viscosity or an optical anisotropy value because they are two ring type compounds. The compounds expressed by formula (8) are used for the object of widening a nematic range by elevating clearing point etc. for the object of lowering threshold voltage and enlarging an optical anisotropy value.
The compounds expressed by formulae (7) to (9) are principally used in the liquid crystal compositions for VA mode requiring a negative dielectric anisotropy value. Increasing the amount makes threshold voltage of the composition small but makes viscosity large. Therefore, it is preferred to use them in a small amount as far as the requirement of threshold voltage is satisfied. However, if the content is less than 40% by weight, voltage driving might be impossible as the absolute value of a dielectric anisotropy value is 5 and less. The amount of the compounds expressed by formulae (7) to (9) is preferably 40% by weight or more in case of preparing the compositions for VA mode, more preferably the range of 50 to 95% by weight. Also the compounds expressed by formulae (7) to (9) are sometimes mixed to the compositions having a positive dielectric anisotropy value for the object of controlling an elastic constant and a voltage-transmission curve of the composition. In this case, the amount of the compounds expressed by formulae (7) to (9) is preferably 30% and less by weight.
As to a third component of the liquid crystal composition of this invention, examples of preferred compounds expressed by formulae (10) to (12) can be illustrated as (10-1) to (10-11), (11-1) to (11-18) and (12-1) to (12-6), respectively. 
wherein R6 and R7 have the same meaning as described above.
In the compounds expressed by formulae (10) to (12), the absolute value of dielectric anisotropy is small and close to a neutral. The compounds expressed by formula (10) are principally used for the object of adjusting viscosity or an optical anisotropy value. And the compounds expressed by formulae (11) and (12) are used for the object of widening a nematic range by elevating a clearing point etc. or adjusting an optical anisotropy value.
Increasing the amount of compounds expressed by formulae (10) to (11) makes threshold voltage of the liquid crystal composition large and makes viscosity small. Therefore, it is desirable to use a large quantity of the compounds as far as the requirement for threshold voltage is fulfilled. In case of preparing the liquid crystal compositions for TFT, the amount of compound expressed by formulae (10) to (12) is preferably 40% and less by weight, more preferably 35% and less by weight. And in case of preparing the liquid crystal compositions for STN or TN, the amount of the compound expressed by formulae (10) to (12) is preferably 70% and less by weight, more preferably 60% and less by weight.
The liquid crystal compositions provided according to this invention preferably comprise at least one liquid crystalline compound expressed by formula (1) in the amount of 0.1 to 99% by weight to have excellent characteristics.
The liquid crystal compositions are generally prepared by known methods, for example, by dissolving various components each other at high temperature. They are optimized and improved, according to an intended usage, by adding appropriate additives, if necessary. Such additives are well known to a person skilled in the art and reported in the literature in detail. Usually a chiral dopant which has effects to prevent a reverse twist by inducing a helical structure in liquid crystals and by adjusting a twist angle to the required value, is added. Examples of the chiral dopant used in this case can be illustrated as follows. 
In the liquid crystal compositions of this invention, their pitch of a twist is usually adjusted by the addition of these optically active compounds. For the liquid crystal compositions for TFT and TN, the pitch of twisting is preferably adjusted in the range of 40 to 200 xcexcm. For the liquid crystal compositions for STN, it is preferably adjusted in the range of 6 to 20 xcexcm. And for the bistable TN mode, it is preferably adjusted in the range of 1.5 to 4 xcexcm. Two or more optically active agents can be added for the purpose of adjusting temperature dependence of the pitch.
The liquid crystal compositions can be used for GH mode by the addition of dichroic dyes such as melocyanines, styryls, azos, azomethines, azoxys, quinophthalones, anthraquinones, or tetrazines. The compositions of this invention can be used for NCAP which is fabricated by microcapsulation of nematic liquid crystals, for polymer dispersion mode liquid crystal display device (PDLCD) which is fabricated by forming three dimensional polymer network in liquid crystals such as polymer network liquid crystal device (PNLCD). Moreover, they also can be used for birefringence control mode (ECB) or DS mode.